In recent years magnetic recording has become the preferred method of storing information such as sound, video images, and computer data. Magnetic tapes and disks are the most common types of recording media. The trend in magnetic recording is towards higher radio frequency outputs at higher frequencies and larger storage capacities. To achieve these objectives, two approaches have been taken: (1) increasing magnetic power by increasing coercive force and remanent magnetic flux, and (2) decreasing self demagnetization by decreasing magnetic layer thickness and optimizing orientation of the magnetic particles.
Individually, each approach appears insufficient to achieve higher radio frequency outputs and larger storage capacities, however, when used in combination, performance is improved significantly to the level of metal evaporative tape.
However, it has been difficult to form thin layer particulate media using conventional coating methods. To deal with this problem, thin layer particulate recording media coated in combination with a nonmagnetic lower-layer have been manufactured using simultaneous wet-on-wet double coating techniques. Such techniques make it possible to get high density particulate media which performs equivalent to metal evaporated tape.
A conventional nonmagnetic lower layer in a multi-layer recording medium is not capable of magnetic recording, but it may supply a smooth bonding surface for the adjacent upper layer and contribute to durability. The use of a nonmagnetic lower layer, however, can result in poor overwrite characteristics. Typical "hard" magnetic lower layers, on the other hand, containing predominantly particles such as acicular Co--.gamma.--Fe.sub.2 O.sub.3, .gamma.--Fe.sub.2 O.sub.3, CrO.sub.2, Fe.sub.3 O.sub.4, and Co--Fe.sub.3 O.sub.4, have high squareness ratios and remanent flux densities. Magnetic media made using such particles in both layers sometimes exhibit unacceptably high levels of demagnetization.
Accordingly, there is a need for an improved multi-layer magnetic medium which exhibits enhanced high frequency output without signal erasure and overwrite problems.
Additionally, there is a need for an improved method for making multi-layer recording media that permits efficient application of successive, multiple layers to a substrate without experiencing significant intermixing of the layers that may be encountered using conventional wet-on-wet techniques.